Electromagnetic (“EM”) telemetry may be used to transmit data from a downhole tool in a wellbore to a receiver at the surface. EM telemetry may be bi-directional with half-duplex transmitters and receivers. EM telemetry may implement a time-sharing schedule between uplink and downlink commands. Real-time (“RT”) data transmission allows for real-time interpretation and decision-making that may be used for steering, well placement, drilling optimization, and safety. The EM telemetry may be subjected to noise from a variety of sources, e.g., power lines, electrical equipment, other EM systems in the area, etc.
To address the noise, a downlink command may be sent to the transmitters to adjust the uplink modulation parameters. The uplink modulation parameters may be adjusted to maximize a signal-to-noise ratio (“SNR”) and minimize power consumed at the transmitters. The uplink modulation parameters may include a modulation type, a carrier frequency, a bandwidth or bitrate, and a signal amplitude for transmission to the surface. When a modulation scheme such as orthogonal frequency-division multiplexing (“OFDM”) is used, the uplink modulation parameters may include a number of subcarriers, subcarrier spacing, and/or cyclic prefix length. To improve reliability, Error Correction Coding (“ECC”) may be used, and the uplink modulation parameters may include an ECC scheme to be used and its coding rate. To determine the uplink modulation parameters, a spectrum of a received signal may be estimated, and the spectrum may be used to derive a noise estimate. Based on the noise estimate, an uplink frequency and bitrate pairs may be determined that predict a desired SNR. This estimation, however, treats the current uplink telemetry signal as noise, in effect, minimizing any frequency bands which overlap a currently selected frequency band.